I am getting this alert repeatedly. Sometimes but not always, it shows when I start Safari. Sometimes it shows when I quit Safari. It also comes up periodically in the Preview app. I'm not entirely sure how to reproduce it, but it happens a few times per day:
Whenever I see the alert, I enter my keychain password, which it seems to accept, but then the same alert will come up again some time later.
I read in this answer that deleting all files and folders in ~/Library/Keychains/ and rebooting might resolve it. I tried this, but I am still getting the prompt repeatedly. I also get it in Preview sometimes. Not sure why. I have entered my Keychain Access password successfully several times.
I contacted Apple Support about this by phone, and they guided me through "Resetting my default keychain" as per this button:
I did that, shut down my computer, turned it back on again, and I am still seeing the alert.
Apple support also guided me through changing my user password under users & groups.
I clicked "change password" and entered a new password.
This had no effect - I am still seeing the alert. I had to enter the new password in order to dismiss the alerts, but they kept coming back again as they did before.
Also, separately, when I go to Keychain Access to view a stored wi-fi password and click on the name of a network in the list from this screen:
I am prompted to enter my Mac OS User password as I expect:
I enter the password successfully, and then it shows the same prompt a second time. I enter the same password again, and then it shows another prompt saying that Mac OS wants to make changes to the System keychain and asks me for an administrator username and password with the username and password fields both blank:
I thought this was strange because the username "Mike Eng" from the earlier prompts is an administrator username. Not sure if that issue is related.
Mac OS 10.12.6